Diagnosis of proliferative sparganosis caused by <i>Spirometra decipiens complex 2</i> ( <i>Spirometra</i> sp. 3) in a domestic cat in the United States

Sparganosis, a zoonotic infection caused by the plerocercoid (sparganum) larval stage of <i>Spirometra</i> spp., is rarely reported in domestic cats. Sparganosis is typically seen as subcutaneous or visceral granulomatous lesions and has been associated with <i>Spirometra mansonoides</i> in North America. A proliferative form of sparganosis, with tissue invasion and widespread dissemination, has always been associated with <i>Sparganum proliferum</i>. However, emerging molecular evidence challenges this distinction. Here, we report a confirmed case of proliferative sparganosis in a domestic cat caused by <i>Spirometra decipiens</i> complex 2 (also referred to as <i>Spirometra</i> sp. 3). The cat had widespread lesions in multiple tissues, with gross and histologic lesions resembling those attributed to <i>S. proliferum</i>. Molecular identification of larval cestode DNA demonstrated a 99% match to <i>S. decipiens</i>, confirming its role in severe disseminated disease. Our case broadens the understanding of the pathogenic capacity of <i>S. decipiens</i> (<i>Spirometra</i> sp. 3) in felids and emphasizes the critical role of molecular detection for accurate species identification. To our knowledge, proliferative sparganosis attributable to <i>Spirometra</i> sp. 3 has not been reported previously in domestic cats. Given the zoonotic potential of sparganosis, our findings have important implications for both veterinary care and public health surveillance.